Space-based remote sensing optical systems are widely used in military, scientific, and commercial applications for viewing the earth and its atmosphere. Space-based remote sensing optical systems typically include a large monolithic primary mirror, one or more smaller monolithic secondary mirrors, and a camera for capturing images.
Space-based remote sensing optical systems that form images of the earth have shown a steady progression toward smaller ground resolution and more spectral information (i.e., a higher degree of color detection or spectral resolution). In the early 1970's, for example, Landsat was able to produce four visible color bands with a ground resolution of about 100 meters in dimension. The next generation of commercial systems will produce hundreds of wavelength bands with a ground resolution of about 1 meter in dimension.
To accommodate the dual needs for small ground resolution and more spectral information, the size of the collecting aperture in the optical system must increase. For example, a Landsat-like orbit of 1,000 km requires a primary mirror aperture of 6 meters to produce a ground resolution of 6 inches. To make hyperspectral images at the one meter pixel size, the radiometrics of the scene and the demand for high signal-to-noise ratio will require an even further increase in the mirror diameter. In addition, any remote sensing system deployed at geosynchronous orbit (.about.36,000 km from earth) will also require larger apertures.
The launch of a monolithic primary mirror large enough to provide the needed ground resolution and spectral information is unlikely due to cost and launch vehicle availability. For these reasons, the primary mirror will likely have to be erected in space after launch using technology such as a rigid or flexible segmented mirror, an electrostatically-controlled membrane mirror, an inflatable primary mirror, an unfurled or rolled mirror, a mirror constructed in space, a sparse aperture mirror, and discrete mirrors in optical communication with one another deployed on separate spacecraft. In each case, the mirror shape will only approximate what is needed to achieve high quality imagery.
In all of these systems, the figure and position of the primary mirror and the position and orientation of the secondary mirror will be somewhat in error, which can significantly adversely impact the operation of the optical system. As used herein, the "figure" of a mirror means the shape of its reflecting surface. After erection, the primary mirror will commonly have a figure which is flawed and be misaligned relative to the secondary mirror. Even when the primary mirror figure and alignment is correct after erection, the mirror figure and alignment will typically change as the spacecraft flies into and out of the earth's shadow (due to the thermal load on the mirror) and as the spacecraft vibrates due to the operation of on-board stabilization equipment. Unlike earth-based mirrors, spaced-based mirrors are typically very flexible and therefore have increased sensitivity to thermal loading and equipment vibration. An incorrectly shaped mirror can cause portions of the wavefront of the radiation to be out of phase with respect to other portions of the wavefront, thereby causing the mirror to work as a series of discrete mirrors rather than as a single mirror. As used herein, "phase" refers to the pathlength of the radiation ray multiplied by 2.pi./.lambda. (where .lambda. is the wavelength of the radiation ray). The rms phase error across the wavefront (i.e., wavefront error) should not exceed about 1 rad, which corresponds to a path length error of about 1.times.10.sup.-7 m (1,000 .ANG.) at the peak visible wavelength to achieve a useful degree of coherence. The required position accuracy of a mirror surface at normal incidence is on the order of this value. "Phase error" is the pathlength error of a radiation ray multiplied by 2.pi./.lambda..
Ground-based telescopes suffer from similar limitations. Modern large astronomical telescopes use thin, flexible mirrors and must sense and reject the effects of wind mechanisms within the telescope and gravitational effects. The latter occur because tracking of stars causes the telescope to tilt, thus changing the gravitational forces on the mirror and telescope structure. For modern astronomical telescopes, the phasing and alignment of the optical system and correction for wavefront errors caused by atmospheric distortions are provided by a guide star or beacon. Radiation from the guide star or beacon can be used to generate electrical signals and align the mirrors and correct for wavefront errors. In earth-viewing optical systems, unlike space-viewing optical systems, there is typically no light source of sufficient intensity and/or duration to permit phasing and alignment of the optical system to occur.